Scotty,
Its not likely but a large DC offset would explain it!
The amps may also be heating up in the rack or your mains sags under load.
You guys have 120 volts , thats twice the current drawn from our 240volts. If you have other heavy appliances or systems on those phases somethingmight be getting quite warm after a while hence the dynamic sack under transients. Whereas the old iron core and big caps hold the charge in the short term. Just a hypothesis .....
Ian